Tube expanding bullet and method of expanding tube

ABSTRACT

A tube expanding bullet is inserted into and expands a heat exchanger tube which has been pierced through collared fin holes of piled heat exchanging fins, so as to expand the fin holes and integrate the tube with the heat exchanging fins. The tube expanding bullet has a first part formed at a front part of the tube expanding bullet and a second part formed at a rear part of the tube expanding bullet whose maximum outer diameter is greater than that of the first part. The fin holes are firstly expanded by passing the first part of the bullet through the tube. The fin hole is secondly expanded by passing the second part of the bullet through the tube. A first rate of expansion of the fin hole by the first part of the tube expanding bullet is less than a second rate of expansion of the fin hole by the second part of the bullet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tube expanding bullet and a method ofexpanding a tube. More particularly, the present invention relates to atube expanding bullet capable of expanding a tube that is piercedthrough collared fin holes of heat exchanging fins so as to expand thefin holes and integrate the tube with the heat exchanging fins, and amethod of expanding a tube with the tube expanding bullet.

2. Description of the Background Art

In heat exchangers of air conditioners, etc., a plurality of heatexchanging fins are integrated with tubes, through which cooling fluidpasses, so as to effectively facilitate heat transfer. A plurality ofheat exchanging fins, each of which has a plurality of fin holes, arepiled to form a heat exchanging body to integrate the heat exchangingfins with the tubes. The tubes are pierced through the fin holes of thepiled heat exchanging fins. Next, the tubes are expanded to integratethe heat exchanging fins with the tubes.

Conventionally, a tube expanding bullet 100 (see FIG. 6) is used toexpand the heat exchanging tubes. The bullet 100 is formed having acolumnar shape, and a maximum outer diameter is equal to an innerdiameter of an expanded tube. The bullet 100 is fixed to a front end ofa mandrel 102 by a cone-shaped member 106.

A male screw section 104, which is upwardly extended from thecone-shaped member 106, is pierced through the bullet 100. The malescrew section 104 is screwed with the mandrel 102 to fix the bullet 100to the mandrel 102.

A through-hole 108 is formed in the cone-shaped member 106. A front endof a screw driver can be inserted into the through-hole 108 so as toeasily detach the bullet 100.

The tube 120 (see FIG. 7) is expanded by the bullet 100 shown in FIG. 6.The tube 120 is pierced through fin holes 116 of heat exchanging fins110 as shown in FIG. 7. A collar 112, whose lower end is bent to form aflange section 114, is extended from an edge of each fin hole 116.

As shown in FIG. 7, the bullet 100, which has a circular projectedsection whose maximum outer diameter is “D”, is inserted into the tube120 which has already been pierced through the fin holes 116. Byinserting the bullet 100, an inner diameter T of the tube 120 isextended to an expanded inner diameter T′, which is equal to the maximumouter diameter D of the bullet 100.

Conventionally, the bullet 100 has the circular projected section whosemaximum outer diameter is D to expand the tube 120 and the fin holes 116and integrate the fins 110 with the tube 120. Inner circumferentialfaces of the collars 112 can be tightly fitted to an outercircumferential face of the tube 120.

However, in FIG. 7, some fins 110 are moved as shown by dotted lines110′ when the bullet 100 is passed through the tube 120. Accordingly,the moved fin 110′ tightly contacts the adjacent fin 110. If some finsare moved and tightly contact adjacent fins, heat exchanging efficiencyof the heat exchanger is lowered.

In order to avoid the tight contact of the fins which occurs by suddenexpansion of the tube, the inventor of Japanese Patent Gazette No.7-124670 invented an improved tube expanding bullet which was disclosedin Japanese Patent Gazette No. 7-124670. The improved bullet 200 isshown in FIG. 8.

The bullet 200 shown in FIG. 8 expands the tube 120 in two stages.Accordingly, the bullet 200 has a front part 202, in which a circularprojected section having a maximum outer diameter A is formed; and arear part 204, in which another circular projected section having amaximum outer diameter B is formed. The relationship between the maximumdiameters A, B and an inner diameter T of the unexpanded tube is T<A<B.

By employing the bullet 200 shown in FIG. 8, the tube 120 is expanded byat least two stages, so that the force expanding the tube 120 can bedispersed evenly along the tube wall. With this dispersion, unlike thecase of expanding the tube 120 with the bullet 100 shown in FIG. 7, theresulting tight contact of the heat exchanging fins 110 can bedecreased.

However, even if the tube is expanded by the bullet 200, the tightcontact of the heat exchanging fins cannot be perfectly prevented. Inthe case of using very thin and light heat exchanging fins, the fins areeven more apt to become tightly contacted and displaced.

In an embodiment of Japanese Patent Gazette 7-124670, a small expandingforce works to an inner face of the collar when the tube is firstlyexpanded and an outer face of the expanded tube lightly contacts theinner face of the collar. With such a small expanding force, thefirstly-expanded tube cannot expand the fin hole, and the collar cannotbe integrated with the firstly expanded tube. In the second expandingstage, the firstly-expanded tube is further expanded and finallyintegrated with the collar. As described hereinabove, the fin whosecollar is expanded by the secondly-expanded tube is displaced andtherefore tightly contacts the adjacent fin whose collar has not yetbeen expanded.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a tube expanding bulletand a method of expanding a tube that are capable of preventing thinheat exchanging fins from becoming displaced and positioned withunfavorably tight contact.

The inventor of the present invention has studied the origin of thetight contact. The present inventor has found that the resulting tightcontact of adjacent fins occurs at a non-expanded portion of the tube aswell.

It appeared that the collar of the fin hole, which is expanded by theexpanded tube, is extended so that the fin is moved and the undesirabletight contact of the adjacent fins occurs.

In the piled fins, as shown in FIG. 7, the flange 114 of each collar 112contacts the lower adjacent fin 110. With this structure, the expandedand extended collar 112 pushes the lower adjacent fin 110 in which thenonexpanded part of the tube is pierced through the fin hole. The lowerfin is not integrated with the tube, so the lower fin is easily moved bythe extended collar 112 of the upper fin and the adjacent fins are movedinto tight contact.

In order to prevent the tight contact of the fins, the present inventorfound that the tube can be properly expanded with a tube expandingbullet such as the two-stage bullet 200 shown in FIG. 8. These and otherobjects of the present invention are accomplished by the steps of:firstly expanding the fin hole by the firstly-expanded tube so as totightly fit the heat exchanging fin to the tube and further expandingthe firstly-expanded tube.

The tube expanding bullet of the present invention is inserted into andexpands a tube which has been pierced through a collared fin hole of aheat exchanging fin so as to expand the fin hole and integrate the tubewith the heat exchanging fin. The tube expanding bullet comprises afirst part being formed at a front part of the tube expanding bullet;and a second part, whose maximum outer diameter is greater than that ofthe first part, being formed at a rear part of the tube expandingbullet.

The fin hole is firstly expanded by passing the first part of the bulletthrough the tube and firstly expanding the tube, and the fin hole issecondly expanded by passing the second part of the bullet through thetube and secondly expanding the tube. The rate of expanding the fin holeby the first part is less than rate of expanding the same by the secondpart.

These and other objects of the present invention are accomplished by amethod comprising the steps of: piercing a tube through collared finholes of a plurality of heat exchanging fins which have been piled; andinserting a tube expanding bullet into the tube so as to expand thetube, whereby the fin holes are expanded by the expanded tube and thetube is integrated with the heat exchanging fins; expanding the tube byinserting the tube expanding bullet so as to expand the fin holes by afirst expansion rate; and secondly expanding the firstly-expanded tubeby the tube expanding bullet by a second expansion rate so as to furtherexpand the expanded fin holes, wherein the first expansion rate of thefin holes is less than the second expansion rate of the fin holes.

The heat exchanging fins are piled and a front end of the collar of eachfin hole contacts an adjacent fin. Then the tube is pierced through thecollar and the fin hole. The tube is expanded, by at least two stages,by inserting the tube expanding bullet into the tube. The expanded tubeexpands the fin hole so as to integrate the fin with the tube. When thetube is expanded, the first part of the bullet, which is formed at thefront part of the bullet, firstly expands the tube, and thefirstly-expanded tube firstly expands the fin hole. By firstly expandingthe tube and the fin hole, the fin can be firstly integrated with thetube.

Then, the second part of the bullet, which is formed at the rear part ofthe bullet, further expands the firstly-expanded tube and thefirstly-expanded fin hole with a greater expansion rate so as to tightlyintegrate the fin with the expanded tube. When the fin hole is secondlyexpanded, the collar is extended and pushes the adjacent fin, but theadjacent fin has already been firstly integrated with the tube. Sincethe fins have been integrated with the tube, no fins are moved, and theundesirable tight contact or fit of the adjacent fins can be prevented.

In the tube expanding bullet and the method of the present invention,the maximum outer diameter of the first part may be designed to make theratio of a first expansion width of the fin hole which has been expandedby the first part of the bullet to the sum of the first expansion widthof the fin hole which has been expanded by the first part and theexpansion width of the fin hole which has been expanded by the secondpart of the bullet 1:10.

And, the tube expanding bullet may further comprise a first circularprojected section, whose maximum outer diameter is equal to that of thefirst part of the bullet formed at the front part of the tube expandingbullet; and a second circular projected section, whose maximum outerdiameter is equal to that of the second part of the tube bullet formedat the rear part of the tube expanding bullet.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings, in which:

FIG. 1 is a front view of the tube expanding bullet of an embodiment ofthe present invention;

FIG. 2 is a partial sectional view of a tube and a fin hole expanded bythe bullet shown in FIG. 1;

FIG. 3A is a sectional view of an unexpanded fin hole;

FIG. 3B is a partial sectional view of the fin holes of piled heatexchanging fins, through which the twice-expanded tube is pierced;

FIG. 4 is a front view of the tube expanding bullet of anotherembodiment of the present invention;

FIG. 5 is a partial sectional view of the tube and the fin hole expandedby the bullet shown in FIG. 4;

FIG. 6 is a front view of a conventional tube expanding bullet;

FIG. 7 is a partial sectional view of a tube and a fin hole expanded bythe conventional bullet shown in FIG. 6; and

FIG. 8 is a partial sectional view of a tube and a fin hole expanded bythe improved bullet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

An embodiment of the tube expanding bullet of the present invention isshown in FIG. 1. The bullet 10 shown in FIG. 1 is capable of expanding atube by two stages, so the bullet 10 has a front (lower) part 18 and arear (upper) part 20, in each of which a circular projected section isformed. The bullet 10, including the front part 18 which acts as a firstexpanding part, and the rear part 20 which acts as a second expandingpart, is fixed to a lower end of a mandrel 12 by a conically-shapedmember 14.

A screw section 16 of the conically-shaped member 14 is pierced throughthe bullet 10 and screws to the lower end of the mandrel 12, so that thebullet 10 can be fixed to the mandrel 12.

A front end of a screw driver can be inserted into a throughhole 22 ofthe conically-shaped member 14 so as to easily detach the bullet 10.

As shown in FIG. 2, the maximum outer diameter “Y” of the circularprojected section of the rear part 20 of the bullet 10 is greater than amaximum outer diameter “X” of the front part 18 thereof. Therelationship between the maximum diameters X, Y and an inner diameter Tof the unexpanded tube is T<X<Y.

The front part 18 of the bullet 10 whose maximum outer diameter is Xexpands the tube 32 by passing the front part 18 of the bullet 10through the tube 32 to produce a first expansion rate. With this firstexpansion rate, the expanded tube 32 expands the fin hole 30. And, therear end part 20 of the bullet 10 whose maximum outer diameter is Y,further expands the firstly-expanded tube 32 and the fin hole 30 with asecond expansion rate. The rate of expansion the fin hole 30 by the rearend part 20 (second expansion rate) is greater than the rate ofexpansion of the fin hole 30 by the front end part 18 (first expansionrate).

As shown in FIG. 2, the tube expanding bullet 10 shown in FIG. 1 isinserted into the tube 32, which has the initial inner diameter T andwhich is pierced through the collars 26. The collars 26 are respectivelyextended from the fin holes 32 and have lower ends formed into flanges28. By inserting the bullet 10, the circular projected section of thefront part 18 firstly expands the tube 32 according to a first expansionrate controlled by the maximum outer diameter X of the tube bullet 10.The firstly-expanded tube 32 pushes an inner face of the collar 26 ofthe fin hole 30 radially outward, so that the fin hole 30 can beexpanded. By this first expansion of the tube 32, the inner face of thecollar 26 of the fin hole 30 can be tightly fitted to thefirstly-expanded tube 32, so that a heat exchanging fin 24 can beintegrated with the firstly-expanded tube 32.

Successively, the circular projected section of the rear part 20 of thetube bullet 10 whose maximum outer diameter is Y, secondly expands thefirstly-expanded tube 32 so that the secondly-expanded tube 32 has thedesired inner diameter T′.

By expanding the tube 32 in two stages, the collar 26 of the fin hole 30can be tightly integrated with the secondly-expanded tube 32, and theheat exchanging fin 24 can be tightly integrated with thesecondly-expanded tube 32.

The bullet 10 is capable of expanding the tube 32 by two stages. A firstrate of expansion of the fin hole 30 by the tube 32 that has beenexpanded in the first stage by the front part 18 (maximum diameter of X)of the tube bullet 10 is less than a second rate of expansion of thefirstly-expanded fin hole 30 by the tube 32 that has been expanded inthe second stage by the rear part 20 (maximum diameter of Y). Namely,the first expansion rate is less than the second expansion rate causedby the varying diameters of the tube bullet 10. Thus, in the firstexpanding stage, the tube 32 can be expanded with a relatively smallexpansion rate, wherein the extension of the collar 26 can be ignored.

The firstly-expanded tube 32 can be secondly expanded to T′ by the rearpart 20 having the maximum diameter Y. In the second expansion stage,the fin hole 30 is further expanded by the secondly-expanded tube 32, sothat the collar 26 of the expanded fin hole 30 is extended. The extendedcollar 26 pushes the adjacent fin.

But, as shown in FIG. 2, parts of the tube 32, which correspond to otherfins, have already been firstly or secondly expanded, so other fins havebeen integrated with the tube 32.

Therefore, other fins are never moved by the collar 32, which isextended in the second expansion stage.

When the fin hole 30 of the fin 24 shown in FIG. 3A is expanded, thecollar 26 of the fin hole 30 is extended. The collar 26 is extended froman edge of an opening section of the fin 24 and the lower end is bent toform the flange 28.

An example of the extension of the collar 26 will be explainedhereinafter. In the present example, a thickness “t” of the heatexchanging fin 24 is 0.1 mm; an inner diameter H of the fin hole 30 is9.9 mm; radii R of a border section between the fin 24 and the collar 26and a border section between the collar 26 and the flange 28 are 0.1 mm;and a length L of the collar 26 is 1.2 mm. In the case of expanding thediameter H of the fin hole 30 from 9.9 mm to 10.0 mm, the length L ofthe collar 26 becomes 1.228 mm. Namely, the collar 26 is extended 0.028mm. If each of the piled fins 24 is similarly extended, the totalextension of 400 fins 24 which are piled is 11.2 mm.

With the extension of the collar 26 which occurs by expanding the finhole 30 and the collar 26, the adjacent fin 24, which has been contactedby the flange 28 of the extended collar 26, and the collar 26 of theadjacent fin 24, are pushed by the extended collar 26.

The action of the extended collar 26 will be explained hereinafter withreference to FIG. 8. The tube expanding bullet 200 shown in FIG. 8expands the tube by two stages. The fin hole 116 of the heat exchangingfin 110 is only expanded by the circular projected section of the rearend part 204 of the bullet 200 whose maximum outer diatmeter is B.Therefore, the extension of the collar 112 a, which occurs by expandingthe fin hole, occurs when the fin hole 116 a of is expanded by the rearend part 204 of the bullet 200.

The pushing force from the extended collar 112 a works to the adjacentfin 110 b, whose flange 114 b contacts the fin 110 a, and anotheradjacent fin 110, which is contacted by the flange 114 a of the collar112 a. But the fin hole 116 b has been expanded by the rear end part 204of the bullet 200, and the collar 112 b is tightly integrated with thetube 120, so that the fin 110 b is not moved by the pushing force of thecollar 112 a.

On the other hand, the fin hole 116 of the fin 110 is not expanded bythe front end part 202 of the bullet 200, and the collar 112 is notintegrated with the tube 120. Therefore, the fin 110 is easily moved bythe pushing force of the collar 112 a, so that the fin 110 is apt totightly fit to another fin.

However, by employing the tube expanding bullet 10 of the presentinvention, the tube 32 and the fin hole 30 of the fin 24 can be expandedin two succesive stages. The expansion rate of the first stage is lessthan that of the second stage. Therefore, the bad influence by theextension of the collar 26, which occurs by expanding the fin hole 30,mainly occurs when the fin hole 30 is expanded in the second stage.

The inner face of the collar 26 a of the fin hole 30 a, which has beenfirstly expanded by the front part 18 of the bullet 10, is tightlyintegrated with the firstly-expanded tube 32. Further, the inner face ofthe collar 26′ of the secondly-expanded fin hole 30′ is also tightlyintegrated with the secondly-expanded tube 32. Therefore, the collar26′, whose flange 30′ contacts the fin 24 b having the secondly-expandedfin hole 26 b, and the fin 24 a, which is contacted by the flange 28 band the collar 26 b, are tightly integrated with the expanded tube 32.With this structure, even if the fin 24 a and the collar 26′ are pushedby the collar 26 b which is extended when the fin hole 26 b is expandedin the second stage, the fins 24 a and 24′ are never moved, so that thetight contact or fit of the fins can be prevented.

Note that, the extension of the collar 26 b, which occurs by expandingthe fin hole 30 b in the second stage, is absorbed by bending the flange28 b in a direction of an arrow C.

If the first expansion rate of the fin hole 30 is greater, the collar 26a of the fin hole 30 a, which has been firstly expanded, pushes andmoves the fin 24, which has not been integrated with the tube 32. Inorder to prevent this action, the first expansion rate of the fin hole30 should be small, wherein the extension of the collar 26 a can beignored.

The expansion rate of the fin hole 30 will be explained hereinafter withreference to FIG. 3B. In FIG. 3B, a plurality of the piled fins 24 areshown which have the fin holes 30 shown in the FIG. 3A. The tube 32 hasbeen pierced through the fin holes 30 of the fins 24. Next, the tube 32is expanded by the tube bullet 30 shown in FIG. 1, by two stages asshown in FIG. 2 and described hereinabove.

In the example shown in FIG. 3B, an inner diameter of the non-expandedfin hole 30 is H. An inner diameter of the hole 30 a of the fin 24 aafter the first expansion stage which is expanded by the tube 32expanded in the first expanded stage is “H”+“Wa.” The value “Wa” is anexpanded width of the fin hole 30 a which is expanded by thefirstly-expanded tube 32 after the first expansion stage.

Further, an inner diameter of the fin hole 30 b of the fin 24 b afterthe second expansion stage is expanded by the tube 32 that has beenexpanded in the second expansion stage is H+Wa+Wb. The expanded width Wbof the fin hole 30 b is that resulting after expansion by thesecondly-expanded tube 32. Therefore, a total expanded width Wc of thefin hole 30 b, which are expanded by the secondly-expanded tube 32, isthe sum of Wa and Wb.

In the present example, the ratio of the expanded width Wa of the finhole 30 a which has been expanded by the firs expansion stage to the sumWc of the expanded widths of the fin hole 30 b which have been expandedby the first and second expansion stages (Wa;Wc) is 1:10. Thus, theextension of the collar 26 a can be ignored.

Preferably, in the bullet 10, the maximum outer diameter X of the frontend part 18 is designed to make the ratio of the expanded width Wa ofthe fin hole 30 a which has been expanded by the first expansion stage32 to the sum Wc of the expanded width of the fin hole 30 b which hasbeen expanded by the first and second expansion tube 32(Wa:Wc)1:10.

In the case of expanding the inner diameter H of the fin hole 30 of thefin 24 shown in FIG. 3A from 9.9 mm to 10.0 mm by two stages, the bullet10 preferably expands the inner diameter H to 9.91 mm in the firstexpansion stage. In this case, the extension of the collar 26 of the finhole 30, which occurs by first expanding the fin hole 30, can besubstantially ignored.

The bullet 10 shown in FIGS. 1 and 2 has a circular valley part 19formed between the front end part 18 and the rear end part 20 so as toreduce friction between the bullet 10 and the inner face of the tube 32.

Since the bullet 10 has the circular valley part 19, a space 34 isformed between the inner face of the tube 32 which has been firstlyexpanded, and the circular valley part 19. The inner diameter of a partof the expanded tube 32, which corresponds to the circular valley part19, may be reduced by a springback phenomenon where the inner diametersprings back into the unexpanded position.

Therefore, in the case of expanding the tube whose material is apt tomake the spring-back phenomenon, a tube expanding bullet 50 shown inFIG. 4 is capable of effectively preventing the spring-back phenomenon.

The bullet 50 shown in FIG. 4 is pierced by the screw section 16 of thecone-shaped member 14. The screw section 16 is screwed with the front(lower) end of the mandrel 12 so that the bullet 50 can be fixed to themandrel 12 as well as the bullet 10 shown in FIG. 1.

The bullet 50 has a front end part 52 and a rear end part 54. The rearend part 54 includes a circular projected section whose maximum outerdiameter is Y (see FIG. 5).

On the other hand, the front end part 52 includes a cylindrical section,which is extended from a border between the front end part 52 and therear end part 54, and a tapered section, which is extended from thelower end of the cylindrical section and whose diameter is graduallyreduced toward the conically-shaped member 14. The maximum outerdiameter of the front end part 52 is equal to an outer diameter X (seeFIG. 5) of the cylindrical section.

The bullet 50 shown in FIG. 5 does not have a circular valley partbetween the front end part 52 and the rear end part 54.

Note, that the bullet 50 firstly expands the tube by the cylindricalsection and a part 56.

In the bullet 50, the diameter X of the cylindrical section of the frontend part 52 is less than the maximum outer diameter Y of the circularprojected section of the rear end part 54. The relationship between themaximum diameters X, Y and the inner diameter T of the unexpanded tubeis T<X<Y.

The outer diameter X of the cylindrical section of the front end part 52is designed to expand the tube 32 in a first expansion stage. When thecylindrical section of the front end part 52 of the bullet 50 isinserted into the tube 32 and expands the tube 32 in the first expansionstage, the fin hole 30 can be firstly expanded by the firstly expandedtube 32 (see FIG. 5).

The diameter X is less than the diameter Y. Further, the rate ofexpansion of the fin hole 30 by the tube 32 expanded by the firstexpansion stage is less than the rate of expansion of thefirstly-expanded fin hole 30 by the tube 32 expanded by the secondexpansion stage (expanded by the rear end part 54 of the bullet 50).

By inserting the bullet 50 shown in FIG. 4 into the tube 32, the finhole 30 is firstly expanded by the firstly-expanded tube 32 in a firstexpansion stage (by the part 56 of the front end part 52), so that thefirstly-expanded collar 26 a can be tightly integrated with thefirstly-expanded tube 32 (see FIG. 5).

The firstly-expanded tube 32 is secondly expanded by the circularprojected section in a second expansion stage (maximum outer diameter Yof circular projected section of the rear end part 54), so that the finhole 30 a is expanded by the secondly-expanded tube 32 in the secondexpansion stage.

As shown in FIG. 5, the cylindrical section of the front end part 52 isfitted to the inner face of the firstly-expanded tube 32, and the space34 (see FIG. 2) is not formed. Even if the material of the tube 32 isapt to experience the so-called spring-back phenomenon, the bullet 50 iscapable of smoothly expanding the firstly-expanded tube 32 withoutpermitting the spring-back phenomenon.

In the above described embodiments, the tube expanding bullet expandsthe tube by two stages. However, the bullet may have three or morecircular projected sections so as to expand the tube by three stages ormore stages. Accordingly, the number of expanding stage is not limited.

In the embodiments, the tube is expanded by one bullet which is capableof expanding by two stages. But the tube may be expanded by a firstbullet for firstly expanding to a first expansion stage and a secondbullet for secondly expanding in a second expansion stage. Namely, thefirst bullet is firstly inserted into the tube so as to firstly expandthe tube, then the second bullet is inserted into the firstly-expandedtube so as to expand the firstly-expanded tube to a second diameter.

The tube expanding bullet may be used to expand the heat exchangingtubes for room air conditioners, heat exchanging units of chemicalplants, etc.

By employing the present invention, the tight contact or fit of theadjacent fins can be prevented, and rate of producing bad heat exchangercomponents can be reduced.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof The presentembodiments are therefore to be considered in all respects asillustrated and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A tube expanding bullet for inserting into andexpanding a tube, the tube being pierced through a fin hole of a heatexchanging fin having a plurality of collars, said tube expanding bulletcomprising: a first expansion part formed at a front end of said tubeexpanding bullet, said first expansion part having a diameter greaterthan an outer diameter of said tube for integrally connecting said tubeto said collars; and a second expansion part formed at a rear end ofsaid tube expanding bullet, said second expansion part having a maximumouter diameter greater than a maximum outer diameter of said firstexpansion part, wherein a first rate of expansion of the fin hole bysaid first expansion part is less than a second rate of expansion bysaid second expansion part.
 2. The tube expanding bullet according toclaim 1, wherein the maximum outer diameter of said first expansion partof said tube expanding bullet imparts a first expanded width of the finhole and said second expansion part of said tube expanding bulletimparts a second expanded width of the fin hole.
 3. The tube accordingto claim 2, wherein the first expanded width of the fin hole relates toa sum of the first expanded width of the fin hole which has beenexpanded by said first expansion part and the second expanded width ofthe fin hole which has been expanded by said second expansion part by aratio of 1:10.
 4. The tube expanding bullet according to claim 1,further comprising: a first circular projected section having a maximumouter diameter equal to the maximum outer diameter of said firstexpansion part at the front end of said tube expanding bullet; and asecond circular projected section having a maximum outer diameter equalto the maximum outer diameter of said second expansion part formed atthe rear end of said tube expanding bullet.
 5. The tube expanding bulletaccording to claim 4, wherein the maximum outer diameter of said firstexpansion part of said tube expanding bullet imparts a first expandedwidth of the fin hole and said second expansion part of said tubeexpanding bullet imparts a second expanded width of the fin hole, thefirst expanded width of the fin hole relates to a sum of the firstexpanded width of the fin hole which has been expanded by said firstexpansion part and the expanded width of the fin hole which has beenexpanded by said second expansion part by a ratio of 1:10.
 6. A methodof expanding a tube for a heat exchanger comprising the steps of:piercing said tube through a plurality of collared fin holes of aplurality of piled heat exchanging fins; inserting a tube expandingbullet into said tube to expand said tube; expanding said tube in afirst expansion stage by inserting said tube expanding bullet so as toexpand the fin holes to a first expansion width and integrallyconnecting said tube with said collared fin holes; and expanding saidtube expanded in said first expansion stage in a successive secondexpansion stage by said tube expanding bullet to secure said expandedtube with said heat exchanging fins and to impart to a second expansionwidth to said heat exchanging fins, wherein a first rate of expansioncorresponding to said first expansion stage is less than a second rateof expansion corresponding to said second expansion stage.
 7. The methodaccording to claim 6, wherein said tube expanding bullet includes afirst expansion part formed at a front part of said tube expandingbullet, and a second expansion part having a maximum outer diametergreater than a maximum outer diameter of said first expansion partformed at a rear part of said tube expanding bullet.
 8. The methodaccording to claim 6, wherein a ratio of the maximum outer diameter ofsaid first expansion part to a sum of the maximum outer diameter of saidfirst expansion part and the maximum outer diameter of the secondexpansion part is 1:10.
 9. The method according to claim 6, wherein themaximum outer diameter of said first expansion part of said tubeexpanding bullet imparts a first expanded width of the fin hole and saidsecond expansion part of said tube expanding bullet imparts a secondexpanded width of the fin hole, the first expanded width relates to asum of the first expanded width and the second expanded width by a ratioof 1:10.
 10. The method according to claim 6, wherein said tubeexpanding bullet includes a first circular projected section formed at afront part of said tube expanding bullet; and a second circularprojected section formed at a rear part of said tube expanding bullet, amaximum outer diameter of said second circular projected portion isgreater than a maximum outer diameter of said first circular projectedsection.